Transfer of Emergency Services Session Between Disparate Subsystems

ABSTRACT

Methods and nodes for coordinating communication sessions are described herein. A method includes providing, by a control system, a signaling anchor point at a domain transfer function in a visited multimedia subsystem for an access signaling leg and a remote access signaling leg for a communication session between a user element and a public service access point and coordinating call signaling for the communication session via the access signaling leg and the remote signaling leg.

This application is a continuation of U.S. application Ser. No.13/675,788, entitled “TRANSFER OF EMERGENCY SERVICES SESSION BETWEENDISPARATE SUBSYSTEMS”, filed Nov. 13, 2012, which is a continuation ofU.S. application Ser. No. 11/760,199, of the same title, filed Jun. 8,2007, now U.S. Pat. No. 8,331,961, which claims the benefit of U.S.Provisional Patent Application No. 60/812,814, entitled “VCC FOR IMS ANDCS EMERGENCY CALLS”, filed Jun. 12, 2006, all of which are incorporatedherein by reference in their entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to communications, and in particular toproviding a multimedia subsystem control for supporting emergencyservices sessions over circuit-switched subsystems and packetsubsystems, as well as effecting transfers of established calls from onesubsystem to another.

2. Description of the Related Art

Voice sessions or calls can be supported by circuit-switched andpacket-based communications. In many communication environments,circuit-switched communications are provided by circuit-switchedsubsystems while packet communications are provided by packetsubsystems. Traditionally, circuit-switched communications have beenused for voice communications, and as such, access to emergency servicesfor police, fire, and ambulance services has been provided bycircuit-switched subsystems.

As voice services are being deployed over packet subsystems, there is aneed to provide support for emergency services for packet subsystemusers. Given the existing emergency services infrastructure in thecircuit-switched subsystems, network providers are employing techniquesto allow the emergency services provided in the circuit-switchedsubsystems to be accessed by the packet subsystem users.

In wireless communication environments, user elements are mobile, and assuch, often move from one location to another. With movement, servicefor the user element may be transferred from one access point or basestation to another within a given subsystem. Many user elements may beserved by different types of subsystems, and based on the location ofthe user elements or a desired application, may transfer service fromone type of subsystem to another.

For emergency services, it is important to maintain an emergencyservices session between an operator and the user element as the userelement transfers from one access point or base station within aparticular subsystem as well as between different subsystems. Althoughtransfers from one access point or base station to another are handledwithin the given subsystem, there is a need to efficiently andeffectively transfer existing emergency services sessions from one typeof subsystem to another.

SUMMARY

The present invention provides session control for an emergency servicessession (E-session) in a multimedia subsystem (MS), such as the InternetProtocol (IP) Multimedia Subsystem (IMS) as defined by the ThirdGeneration Partnership Project (3GPP). The MS resides in one or morepacket based subsystems (PSs), and E-sessions may include emergencyservices calls, such as 911 calls, that are directed to a PublicServices Access Point (PSAP) that supports emergency services. Sessioncontrol of the E-sessions originating in a circuit-switched subsystem(CS) or the MS is provided by the visited MS and anchored at a domaintransfer function (DTF), which is a service provided by a visited MS.The visited MS is the MS associated with the CS or PS that is currentlyserving the user element. As such, all call signaling for the E-sessionis passed through the DTF. The DTF enables efficient and effectivesubsystem transfers of the E-session between the CS and MS, whilemaintaining the E-session between the user element and the PSAP. The DTFmay also provide to the PSAP location information bearing on thelocation of the user element when the E-session is established, as wellas provide updated location information when transfers between the CSand MS occur.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a communication environment illustrating multimedia subsystemaccess for a user element according to a one embodiment of the presentinvention.

FIG. 2 is a communication environment illustrating circuit-switchedsubsystem access for a user element according to one embodiment of thepresent invention.

FIGS. 3A-3C depict a communication flow illustrating originating anemergency services session via a multimedia subsystem according to oneembodiment of the present invention.

FIGS. 4A and 4B depict a communication flow illustrating the transfer ofthe call established in FIGS. 3A-3C to the circuit-switched subsystemaccording to one embodiment of the present invention.

FIG. 5 is a communication flow illustrating the transfer of the call ofFIGS. 4A and 4B back to the multimedia subsystem according to oneembodiment of the present invention.

FIG. 6 is a communication environment illustrating multimedia subsystemaccess for a user element according to another embodiment of the presentinvention.

FIG. 7 is a communication environment illustrating circuit-switchedsubsystem access for a user element according to another embodiment ofthe present invention.

FIG. 8 is a block representation of a service node according to oneembodiment of the present invention.

FIG. 9 is a block representation of a user element according to oneembodiment of the present invention.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

The present invention provides session control for an emergency servicessession (E-session) in a multimedia subsystem (MS), such as the InternetProtocol (IP) Multimedia Subsystem (IMS) as defined by the ThirdGeneration Partnership Project (3GPP). The MS resides in one or morepacket based subsystems (PSs), and E-sessions may include emergencyservices calls, such as 911 calls, that are directed to a PublicServices Access Point (PSAP) that supports emergency services. Sessioncontrol for E-sessions originating in a circuit-switched subsystem (CS)or the MS is provided by the visited MS and is anchored at a domaintransfer function (DTF), which is a service provided by a visited MS.The visited MS is the MS associated with the CS or PS that is currentlyserving the user element. As such, all call signaling for the E-sessionis passed through the DTF. The DTF enables efficient and effectivesubsystem transfers of the E-session between the CS and MS, whilemaintaining the E-session between the user element and the PSAP. The DTFmay also provide to the PSAP location information bearing on thelocation of the user element when the E-session is established, as wellas provide updated location information when transfers between the CSand MS occur.

As an E-session is being initiated, the DTF is inserted into thesignaling path of the E-sessions by an emergency call/session controlfunction (E-CSCF) in the visited MS. The E-CSCF functions as a speciallyconfigured serving CSCF (S-CSCF) of an IMS. In general, CSCFs are oftenSession Initial Protocol (SIP) proxies that provide various sessioncontrol functions at various points in the session signaling path. TheE-CSCF provides routing and translation functions for E-sessions, andnotably, invokes services including the DTF when E-sessions are beingestablished. To act as an anchor for the signaling path of an E-session,the DTF may employ a back-to-back user agent function. As such, when theuser element originates an E-session, the DTF will terminate an accesssignaling leg from the user element and establish a remote signaling legtoward the PSAP. Subsequently, the DTF will coordinate call signalingbetween the access signaling leg and the remote signaling leg for theE-session. Notably, the user element may provide location informationbearing on the location of the user element for delivery to the DTF,which will pass the location information to the PSAP.

Subsystem transfers enable the user element to move back and forthbetween the CS and the MS while maintaining an active E-session.Subsystem transfers associated with a given E-session, including initialand subsequent subsystem transfers, are executed and controlled in thevisited MS by the DTF, generally upon a transfer-specific request or anew E-session received from the user element. Upon detecting conditionsrequiring a transfer from one subsystem to another, the user elementwill establish a new access signaling leg with the DTF using anappropriate address for the DTF or an emergency address, as if a newE-session were being established. The user element may provide updatedlocation information to the DTF when establishing the new access leg.The new access signaling leg is established via the “transferring-in”subsystem and establishment of the new access signaling leg will triggera transfer from the “transferring-out” subsystem to the transferring-insubsystem. The DTF will implement a subsystem transfer by replacing theold access signaling leg currently communicating with the remotesignaling leg with the new access signaling leg established via thetransferring-in subsystem. The DTF will also send an update toward thePSAP to provide information necessary to facilitate a transfer of thebearer path to the transferring-in subsystem. The DTF may also provide alocation update for the user element to the PSAP, if a location updateis available. The DTF will subsequently release the old access signalingleg that was established through the “transferring-out” subsystem.

The switch of the access signaling legs from the transferring-outsubsystem to the transferring-in subsystem does not impact the remotesignaling leg or the application services, including the DTF, in theremote signaling leg. Using the access signaling leg in thetransferring-in subsystem and the remote signaling leg, the appropriatebearer path may be established to the user element via an appropriate CSclient or MS client of the user element. Since all call signaling isprovided through the DTF, additional services may be associated with thecall through any number of transfers.

For routing of emergency calls originating via circuit-switchedsubsystem and for subsystem transfers, the DTF is addressable from thedifferent subsystems using different service identities (SI). In the CS,a subsystem transfer directory number (STN) associated with the DTF maybe used for routing call signaling messages from within the CS towardthe DTF in the MS. In the MS, a subsystem transfer uniform resourcelocation (STU) associated with the DTF is used for routing callsignaling messages along the access signaling leg toward the DTF. In oneembodiment, the STN and the STU are provided to the user element whenthe E-session is being established for the first time. In the followingdescription, 3GPP TS 24.008 (DTAP) is used in the CS, while the SessionInitiation Protocol (SIP) is used in the MS to effect origination,termination, and transfer of calls. Those skilled in the art willrecognize other applicable and useful protocols as substitutes for DTAPand SIP.

With reference to FIG. 1, a wireless communication environment isillustrated with a visited MS 10, and a CS-based PSAP domain 12. Asdepicted, a user element (UE) 14 has established an E-session via thevisited MS 10 into the PSAP domain 12. The signaling path for theE-session includes an access signaling leg and a PSAP signaling leg. Theaccess signaling leg extends from the user element 14 through aproxy-CSCF (P-CSCF) 16, an E-CSCF 18, and to a DTF 20, where the sessionsignaling is anchored. The PSAP signaling leg extends from the DTF 20through the E-CSCF 18, to a voice over IP (VoIP) positioning center(VPC) 22, back through the E-CSCF 18, through a PSAP media gatewaycontroller (MGC) 24, to a PSAP 26, which is in the PSAP domain 12.Notably, the DTF 20 and the VPC 22 are applications that are closelyassociated with the E-CSCF 18. The bearer path for the E-session extendsfrom the user element 14 to a PSAP media gateway (MG) 28, which isassociated with the PSAP MGC 24, to the PSAP 26.

The P-CSCF 16 is typically the signaling entry point for the visited MS10, and as such, facilitates routing of an E-session to an appropriateE-CSCF 18 in the visited MS 10. When an E-session is initiated, theE-CSCF 18 will invoke the DTF 20, effectively insert the DTF 20 in thesession signaling path, and then cooperate with the DTF 20 to route theE-session toward the PSAP 26. During this routing, the E-CSCF 18 mayalso invoke the VPC 22, such that both the DTF 20 and the VPC 22 areinserted into the signaling path. Since the PSAP 26 is in a CS-basedPSAP domain 12 in this example, the PSAP MGC 24 is employed to route thecall to the PSAP 26 and establish a bearer path via the PSAP mediagateway 28.

The VPC 22 helps the E-CSCF 18 to determine an appropriate PSAP 26,assuming there are multiple PSAPs 26 available. Preferably, the PSAP 26selected for a given E-session is determined based on the location ofthe user element 14. Since the user element 14 may send a locationreference with the initiation of an E-session, the VPC 22 will accessthe location reference, identify a location of the user element 14 basedon the location reference, and locate an appropriate PSAP 26 to use forthe E-session.

In one embodiment, the VPC 22 will determine an emergency servicesrouting number (ESRN) for the selected PSAP 26, and a routing keyassociated with the E-session. The E-CSCF 18 will use the ESRN to routethe E-session toward the selected PSAP 26. The routing key is used as areference for the E-session and any contextual information associatedwith the E-session. The routing key is helpful in associating additionalsignaling or session requests associated with the E-session. Notably, alocation information service (LIS) 30 may be accessible by the VPC 22 toobtain PSAP and ESRN information based on location informationassociated with the user element 14.

For the present invention, the DTF 20 is a new function that is closelyassociated with, if not integrated with, the E-CSCF 18. The DTF 20 isinserted into the signaling path when an E-session is established, suchthat the DTF 20 can control subsequent subsystem transfers between a CSand the visited MS 10, and in particular, a PS supporting the visited MS10. Once an E-session is established, the DTF 20 will recognize that anE-session is active for a given user element 14. When a request for anew E-session comes in from a user element 14 that is already engaged inan active E-session with the DTF 20, the DTF 20 will effect a transferfrom a transferring-out subsystem to a transferring-in subsystem fromwhich the new E-session is being initiated. The DTF 20 will effectivelyassociate the PSAP signaling leg, which remains intact during thetransfer, with a new access signaling leg established via thetransferring-in subsystem. Additionally, the DTF 20 will send an updatetoward the PSAP 26 to provide information regarding the new bearer paththrough the transferring-in subsystem, as well as updated locationinformation associated with the user element 14.

With reference to FIG. 2, the signaling and bearer paths are illustratedfor an E-session that was initiated from the user element 14 in avisited CS 22. The access signaling leg extends from the user element 14to the DTF 20 via a visited mobile switching center (VMSC) 34 thatserves the user element 14, a CS MGC 36 that is associated with a CSmedia gateway (MG) 38, an interrogating CSCF (I-CSCF) 40, and a CSadaptation function (CSAF) 42. The bearer path extends from the userelement 14 through the visited CS 32 to the CS MG 38. The bearer path isprovided through the visited MS 10 between the CS MG 38 and the PSAP MG28. The final section of the bearer path through the CS-based PSAPdomain 12 is provided between the PSAP MG 28 and the PSAP 26.

The CS MGC 36 and the CS MG 38 cooperate to provide access between thevisited CS 32 and the visited MS 10. The I-CSCF 40 initially receivesthe E-session and selects the CSAF 42 to use for representing the userelement 14 to a selected E-CSCF 18. The CSAF 42 effectively emulates thebehavior of the user element 14, if the user element 14 were served bythe visited MS 10. Accordingly, the CSAF 42 provides a remote user agentfunction on behalf of the user element 14, when the user element 14 isserved by the visited CS 32. The E-CSCF 18 will invoke the DTF 20 toanchor the access signaling and PSAP signaling legs for the E-session.The E-CSCF 18 will also invoke the VPC 22, as described above, to helpselect an appropriate PSAP 26 and obtain an ESRN or other routinginformation necessary to allow the E-CSCF 18 to route the E-sessiontoward the PSAP 26. In this embodiment, the E-CSCF 18 need not knowwhether the user element 14 is supported by the visited CS 32 or thevisited MS 10, because of the CSAF 42. Notably, a home subscriberservice (HSS) 44 is provided to provide routing information for theI-CSCF 40 and perhaps for the VMSC 34. In this embodiment, the I-CSCF 40may access the HSS 44 to determine a CSAF 42 to which the E-sessionshould be routed.

With reference to FIGS. 3A through 3C, a communication flow is providedto illustrate origination and establishment of an E-session that isoriginated from a user element 14 that is served by the visited MS 10(and thus, is not being served by the visited CS 32). Assuming SIP isused for session signaling within the visited MS 10, the user element 14may initiate an emergency Invite message (E-Invite), which is receivedby the P-CSCF 16 (step 100). The E-Invite may include an indication thatit is an emergency session request, such as by including an emergencyservices number or address (911) and indicate that it is from the userelement 14. The user element 14 may also include a location reference,which provides information bearing on the actual location of the userelement 14. The location reference may relate to a base station, accesspoint, network ID, or the like from which the location of the userelement 14 may be derived. If the user element 14 is equipped withpositioning capability, the actual position of the user element 14 maybe provided as the location reference, alone or in association with theabove-mentioned location indicia.

The P-CSCF 16 will select an E-CSCF 18 and send the E-Invite toward theE-CSCF 18 (step 102), which will forward an Invite to the DTF 20 (step104). Forwarding the Invite to the DTF 20 will effectively invoke theDTF 20 as an anchor point for the signaling path for the E-session. Assuch, the DTF 20 will establish itself as an anchor and take thenecessary steps to complete establishment of the E-session (step 106).The DTF 20 will send an Invite back to the E-CSCF 18 (step 108), whichwill invoke the VPC 22 by sending an Invite to the VPC 22 (step 110).The VPC 22 will identify a location of the user element 14 based on thelocation reference provided in the Invite (step 112), and will select aPSAP 26 based on the location of the user element 14 (step 114). Asnoted above, the VPC 22 may query the LIS 30 to identify the location ofthe user element 14 and select an appropriate PSAP 26. The VPC 22 willcreate a context for the call, and will provide an associated emergencyservices routing key (ESK) associated with the context (step 116). Thecontext maintains a reference to the E-session and any services orinformation related to the user element 14, a subscriber associated withthe user element 14, or services associated with the E-session. Finally,the VPC 22 will identify an emergency services routing number (ESRN) forthe selected PSAP 26 (step 118).

The VPC 22 may then send to the E-CSCF 18 a Re-Direct message, which isconfigured to route the E-session toward the PSAP 26 (step 120). TheRe-Direct message may include the emergency services routing number, theemergency services routing key, and the location reference or locationderived therefrom. In response, the E-CSCF 18 will send an Invite towardthe PSAP 26. In this instance, the Invite is directed to the PSAP MGC24, which is associated with the ESRN and the PSAP 26 (step 122). Inresponse to receiving the Invite, the PSAP MGC 24 will send anIntegrated Services User Part (ISUP) Initial Address Message (IAM)toward the PSAP 26 through the CS in which the PSAP 26 is located (step124). The IAM will include the location reference, such that the PSAP 26can identify the location of the user element 14.

While the E-session is being presented to the PSAP 26, the PSAP MGC 24will send a 183 Session Progress message back toward the E-CSCF 18 toindicate that the call is being presented to the PSAP 26 (step 126).Since the session signaling path may extend through the VPC 22 and theDTF 20, the E-CSCF 18 will send the 183 Session Progress message to theVPC 22 (step 128), which will send the 183 Session Progress message backto the E-CSCF 18 (step 130). Notably, other embodiments will not keepthe VPC 22 in the signaling path; however, the VPC 22 is maintained inthe signaling path for the described embodiment. The E-CSCF 18 will thenroute the 183 Session Progress message through the DTF 20 (steps 132 and134) and then toward the P-CSCF 16 (step 136). The P-CSCF 16 will sendthe 183 Session Progress message to the user element 14 (step 138),wherein the user element 14 may provide an alert to the user that thecall is being presented to emergency services. During this time, assumethat the E-session is processed as indicated by the providing of an ISUPAddress Complete Message (ACM) back toward the PSAP MGC 24 (step 140). Acorresponding SIP message may be routed back toward the user element 14over the signaling path (not shown).

Once the E-session is answered, the PSAP 26 will send an ISUP AnswerMessage (ANM) toward the PSAP MGC 24 (step 142). In response to the ANM,the PSAP MGC 24 will send a 200 OK message to the E-CSCF 18 (step 144),which will route the 200 OK message through the VPC 22 (steps 146 and148), and then to the DTF 20 (step 150). Since the E-session is anchoredat the DTF 20, and the DTF 20 will control subsequent subsystemtransfers, an STN for MS-to-CS transfers and an STU for CS-to-MStransfers may be provided for the user element 14 (step 152). Again, theSTN is a directory number that the user element 14 may use to initiate anew access signaling leg toward the DTF 20 from within the visited CS32. The STU is a uniform resource location (URL) that the user element14 may use to initiate a new access signaling leg toward the DTF 20 fromwithin the visited MS 10, or in other words, from a PS served by thevisited MS 10. The DTF 20 will then forward the 200 OK message with theSTN and STU to the E-CSCF 18 (step 154), which will send the 200 OKmessage toward the user element 14 via the P-CSCF 16 (steps 156 and158). The user element 14 will recognize that the call has been answeredupon receipt of the 200 OK message, and will extract and store the STNand the STU provided in the 200 OK message (step 160). Notably, theinitial Invite may include the Session Data Protocol (SDP) informationof the user element 14 to allow the PSAP MG 28 to deliver bearer trafficto the user element 14. Similarly, the 200 OK message provided by thePSAP MG 28 will provide SDP information to allow the user element 14 todeliver bearer traffic to the PSAP MG 28 to facilitate a bearer path(step 162). Notably, the bearer path between the user element 14 and thePSAP MG 28 is packet-based and supported by the underlying PS, whereinthe CS-based portion of the bearer path between the PSAP MG 28 and thePSAP 26 may be a circuit-switched time division multiplexed (TDM)connection.

With reference to FIGS. 4A and 4B, a communication flow illustrating asubsystem transfer from the visited MS 10 to the visited CS 32 isprovided. Initially, assume the user element 14 determines a need totransfer from the visited MS 10 to the visited CS 32 (step 200).Notably, the E-session established via the communication flow of FIGS.3A through 3C is active and anchored in the DTF 20. To initiate thesubsystem transfer, the user element 14 may send an emergencyorigination message (E-Origination) to the VMSC 34, which is serving theuser element 14 (step 202). Notably, the E-Origination may include theSTN and an updated location reference in case the location of the userelement 14 has changed from the time the E-session was originallyinitiated. In response to the E-Origination message the VMSC 34discovers the STN associated with the E-origination if one is notincluded by the UE and will send an ISUP IAM toward the CS MGC 36 thatis associated with the STN (step 204). Notably, the ISUP IAM willinclude the updated location reference. The CS MGC 36 will generate anInvite, which is delivered into the visited MS 10 and received by theI-CSCF 40 (step 206). The Invite will include the STN, the updatedlocation reference, and the SDP information associated with the CS MG38.

The I-CSCF 40 may access the HSS 44 to identify an CSAF 42 to which thesession request should be routed in light of the STN (step 208). Oncethe CSAF 42 has been identified, the I-CSCF 40 will forward the Inviteto the CSAF 42, which is associated with the E-CSCF 18 (step 210). TheCSAF 42 will act as a remote user agent on behalf of the user element 14(for the CS access signaling leg) and forward the Invite to the E-CSCF18 (step 212). The E-CSCF 18 will forward the Invite to the DTF 20 (step214), which will initiate an MS-to-CS transfer and provide a locationupdate for the PSAP 26 (step 216).

Accordingly, the DTF 20 will send a Re-Invite toward the PSAP 26 toupdate the communication information for the bearer path and provide theupdated location reference, which was provided in the Invite or derivedtherefrom (step 218). The CS communication information may include theSDP of the media gateway, and any other information necessary forsupport of a bearer path between the PSAP MG 28 and the CS MG 38. TheE-CSCF 18 will route the Re-Invite through the VPC 22 (steps 220 and222) and on toward the PSAP MGC 24 (step 224). The PSAP MGC 24 willprocess the CS communication information to enable delivery ofinformation from the PSAP 26 over a new bearer path to the CS MG 38 aswell as provide information as to the updated location reference to thePSAP 26 (step 226). The call will be processed, and a 200 OK message,which is not illustrated, is propagated back toward the user element 14along the signaling path, and will provide the SDP information of thePSAP MG 28 to the CS MGC 36 for processing by the CS MG 38. As s result,a bearer path is established between the user element 14 and the PSAP 26via the CS MG 38 and the PSAP MG 28 (step 228). The bearer path ispacket-based between the CS MG 38 and the PSAP MG 28. A TDM-basedcircuit-switched connection is provided between the PSAP MG 28 and thePSAP 26, as well as between the CS MG 38 and the user element 14 via theVMSC 34.

With reference to FIG. 5, a communication flow is provided fortransferring the E-session from the visited CS 32 back to the visited MS10. Initially, the user element 14 will determine a need to transferfrom the visited CS 32 to the visited MS 10 (step 300). The user element14 will initiate an E-Invite toward the DTF 20, potentially using theSTU. The E-Invite will include an updated location reference in case theuser element 14 has moved since the last transfer. The P-CSCF 16 willreceive the Invite from the user element 14 (step 302), and forward theE-Invite to the E-CSCF 18 (step 304). The E-CSCF 18 will forward anInvite to the DTF 20 (step 306), which will initiate a subsystemtransfer from the visited CS 32 to the visited MS 10, and provide alocation update for the PSAP 26 (step 308). Accordingly, the DTF 20 willsend a Re-Invite with communication information for the visited MS 10and the updated location reference to the E-CSCF 18 (step 310). Thecommunication information for the visited MS 10 may include the SDPinformation for the user element 14. The E-CSCF 18 will then route theRe-Invite through the VPC 22 (step 312 and 314), and then to the PSAPMGC 24 (step 316). The PSAP MGC 24 may send an Information messageproviding the updated location reference to the PSAP 26 (step 318). Atthis point, the PSAP MGC 24 will have access to the SDP information ofthe user element 14, and will ultimately provide a 200 OK message (notshown) in response to the Re-Invite back toward the user element 14 overthe signaling path. In the 200 OK message, the SDP information of thePSAP MG 28 is provided to the user element 14. With the exchange of theSDP information between the user element 14 and the PSAP MG 28, thepacket portion of a bearer path may be established between the userelement 14 and the PSAP MG 28 (step 320). The TDM-based CS connectionbetween the PSAP MG 28 and the PSAP 26 will remain intact and will beinterworked with the packet portion of the bearer path via the PSAP MG28.

From the above, the DTF 20 may recognize that an E-session is alreadyestablished with the user element 14 in one domain when a new requestfor an E-session is received from the user element 14 in another domain.In response, the DTF 20 will effect a transfer to the subsystem fromwhich the new session is being requested. In effect, the remote accessleg is maintained while a new access signaling leg is establishedthrough the subsystem from which the new E-session is requested. The DTF20 will associate the new access signaling leg with the remote accesssignaling leg, and release the old access signaling leg to effect thetransfer from one subsystem to another. During this process, any updatedlocation information provided by the user element 14 or associated withthe user element 14 may be forwarded toward the PSAP 26 by the DTF 20.

In the above embodiments, assume that the PSAP domain 12 and the PSAP 26therein are served by a CS. As illustrated in FIGS. 6 and 7, the PSAPdomain 12, like the visited MS 10, may be served by a PS. As such, thereis no need for a media gateway, such as the PSAP MG 28, to provideinterworking between the PS that supports the visited MS 10 and a CSsupporting the PSAP domain 12. When the PSAP domain 12 is supported by aPS, packet-based communications may take place directly or indirectlybetween the E-CSCF 18 and the PSAP 26. Similarly, the bearer path may becompletely packet based from the user element 14 to the PSAP 26, asillustrated in FIG. 6, or between the CS MG 38 and the PSAP 26, asillustrated in FIG. 7.

With reference to FIG. 8, a service node 46 is provided according to oneembodiment of the present invention. The service node 46 may reside inthe visited MS 10 and include a control system 48 and associated memory50 to provide the functionality for any one or a combination of thefollowing: the P-CSCF 16, the I-CSCF 40, the E-CSCF 18, the CSAF 42, theDTF 20, the VPC 22, or any combination thereof. The control system 48will also be associated with a communication interface 52 to facilitatecommunications with any entity affiliated with the visited MS 10 orappropriately associated networks.

With reference to FIG. 9, a block representation of a user element 14 isprovided. The user element 14 may include a control system 54 havingsufficient memory 56 to support operation of a CS client 58 and an MSclient 60, which support CS and PS communications, respectively. Thecontrol system 54 will cooperate closely with a communication interface62 to allow the CS client 58 and the MS client 60 to facilitatecommunications over a CS or the PS (MS) as described above. The controlsystem 54 may also be associated with a user interface 64, which willfacilitate interaction with the user. The user interface 64 may includea microphone and speaker to facilitate voice communications with theuser, as well as a keypad and display to allow the user to input andview information to support media sessions and control of the userelement 14.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A method comprising: providing, by a controlsystem, a signaling anchor point at a domain transfer function in amultimedia subsystem for a first access signaling leg and a remoteaccess signaling leg for an emergency services session between a userelement and a public service access point (PSAP), wherein the domaintransfer function is inserted in the signaling path to anchor the firstaccess leg signaling leg and the remote signaling leg; wherein the firstaccess signaling leg is provided toward the user element via atransferring-out subsystem; establishing, by the control system, theemergency services session in response to receiving a session initiationmessage originating from the user element, wherein the sessioninitiation includes location information for the user element;coordinating, by the control system, call signaling for thecommunication session via the first access signaling leg and the remotesignaling leg; receiving, by the control system, a session initiationmessage responsive to initiation of a new communication session via atransferring-in subsystem to effect a transfer from a transferring-outsubsystem to the transferring-in subsystem; detecting, by the controlsystem, an existence of the communication session with the user elementwhen the session initiation message is received; and associating, by thecontrol system, a second access signaling leg with the remote accesssignaling leg, and releasing the first access signaling leg session inresponse to the detecting the existence of the communication session. 2.The method of claim 1, further comprising sending, by the controlsystem, an update message on the remote leg toward the PSAP to updatethe communication information for the bearer path.
 3. The method ofclaim 1, further comprising establishing, by the control system, thesecond access signaling leg for the communication session.
 4. The methodof claim 1, further comprising coordinating, by the control system, callsignaling exchanges for the communication session via the second accesssignaling leg and the remote signaling leg after the transfer to thetransferring-in subsystem.
 5. The method of claim 1, further comprisingestablishing, by the control system, the remote signaling leg for theemergency services session toward the PSAP.
 6. The method of claim 1,wherein the transferring-out subsystem is a packet based subsystem, andthe transferring-in subsystem is a circuit-switched subsystem.
 7. Themethod of claim 1 further comprising selecting, by the control system,the public service access point based on the location information forthe user element.
 8. A non-transitory computer-readable medium storingsoftware instructions executable by a multimedia subsystem forcoordinating an emergency services session, wherein the softwareinstructions are configured to, when executed, cause the multimediasubsystem to: provide a signaling anchor point for a first accesssignaling leg and a remote access signaling leg for the emergencyservices session between a user element and a public service accesspoint (PSAP), wherein the domain transfer function is inserted in thesignaling path to anchor the first access leg signaling leg and theremote signaling leg; wherein the first access signaling leg is providedtoward the user element via a transferring-out subsystem; establish theemergency services session in response to receiving a session initiationmessage originating from the user element, wherein the sessioninitiation includes location information for the user element;coordinate call signaling for the communication session via the firstaccess signaling leg and the remote signaling leg; receive a sessioninitiation message responsive to initiation of a new communicationsession via a transferring-in subsystem to effect a transfer from atransferring-out subsystem to the transferring-in subsystem; detect anexistence of the communication session with the user element when thesession initiation message is received; and associate a second accesssignaling leg with the remote access signaling leg, and releasing thefirst access signaling leg session in response to the detecting theexistence of the communication session.
 9. The non-transitorycomputer-readable medium of claim 8, wherein the software instructionsare further configured to cause the multimedia subsystem to send anupdate message on the remote leg toward the PSAP to update thecommunication information for the bearer path.
 10. The non-transitorycomputer-readable medium of claim 8, wherein the software instructionsare further configured to cause the multimedia subsystem to establishthe second access signaling leg for the communication session.
 11. Thenon-transitory computer-readable medium of claim 8, wherein the softwareinstructions are further configured to cause the multimedia subsystem tocoordinate call signaling exchanges for the communication session viathe second access signaling leg and the remote signaling leg after thetransfer to the transferring-in subsystem.
 12. The non-transitorycomputer-readable medium of claim 8, wherein the software instructionsare further configured to cause the multimedia subsystem to establishthe remote signaling leg for the emergency services session toward thePSAP.
 13. The non-transitory computer-readable medium of claim 8,wherein the transferring-out subsystem is a packet based subsystem, andthe transferring-in subsystem is a circuit-switched subsystem.
 14. Thenon-transitory computer-readable medium of claim 8, wherein the softwareinstructions are further configured to cause the multimedia subsystem toselect the public service access point based on the location informationfor the user element.
 15. A node comprising: a communication interface;and a control system associated with the communication interface andconfigured to: provide a signaling anchor point at a domain transferfunction in a multimedia subsystem for a first access signaling leg anda remote access signaling leg for the emergency services session betweena user element and a public service access point (PSAP), wherein thedomain transfer function is inserted in the signaling path to anchor thefirst access leg signaling leg and the remote signaling leg; wherein thefirst access signaling leg is provided toward the user element via atransferring-out subsystem; establish the emergency services session inresponse to receiving a session initiation message originating from theuser element, wherein the session initiation includes locationinformation for the user element; coordinate call signaling for thecommunication session via the first access signaling leg and the remotesignaling leg; receive a session initiation message responsive toinitiation of a new communication session via a transferring-insubsystem to effect a transfer from a transferring-out subsystem to thetransferring-in subsystem; detect an existence of the communicationsession with the user element when the session initiation message isreceived; and associate a second access signaling leg with the remoteaccess signaling leg, and releasing the first access signaling legsession in response to the detecting the existence of the communicationsession.
 16. The node of claim 15, the node further configured to sendan update message on the remote leg toward the PSAP to update thecommunication information for the bearer path.
 17. The node of claim 15,the node further configured to establish the second access signaling legfor the communication session.
 18. The node of claim 15, the nodefurther configured to coordinate call signaling exchanges for thecommunication session via the second access signaling leg and the remotesignaling leg after the transfer to the transferring-in subsystem. 19.The node of claim 15, the node further configured to establish theremote signaling leg for the emergency services session toward the PSAP.20. The node of claim 15, wherein the transferring-out subsystem is apacket based subsystem, and the transferring-in subsystem is acircuit-switched subsystem.